📝 STANDARD DESCRIPTION:
Type: Extrinsic, Deep Layer
Origin: Thoracolumbar fascia, spinous processes of lumbar/caudal thoracic vertebrae, last few ribs
Insertion: Teres major tuberosity on medial humerus (common tendon with teres major)
Function: Primary retractor of free limb; power stroke of propulsion (draws trunk forward when limb fixed); antagonist to brachiocephalicus.
📚 DETAILED DESCRIPTION:
THE POWER STROKE ENGINE
A broad, triangular sheet covering the dorsal trunk, representing the primary caudal pulling force (retractor) of the forelimb and the essential 'power stroke' muscle for propulsion.
ORIGIN COMPLEX: Extensive origin from thoracolumbar fascia (lumbodorsal fascia), spinous processes of lumbar vertebrae and caudal thoracic vertebrae, plus the last 2-3 ribs. This wide origin allows it to harness power from the entire caudal trunk.
TENDON INSERTION: Inserts via a strong tendon common with the teres major on the teres major tuberosity (medial aspect of humeral shaft). This shared insertion creates a powerful adductor/retractor complex.
FUNCTIONAL biomechanics: When the limb is not bearing weight, it pulls the humerus (and thus the limb) caudally (retraction). When the limb is fixed during the stance phase, it draws the trunk forward over the planted limb—this is the fundamental propulsive mechanism for forward movement. It is the direct antagonist to the brachiocephalicus.
CARDINAL IMPORTANCE: Without this muscle, the animal cannot generate forward momentum from the forelimb, though it can still bear weight.
// COMPARATIVE ANALYSIS
| HORSE |
Similar origin and insertion; equally crucial for propulsion. |
| DOG |
Insertion varies slightly; less dominant due to weight distribution differences. |
CLINICAL RELEVANCE
Thoracodorsal nerve Injury: Results in inability to pull trunk forward effectively, causing a shortened stride and trunk 'lurching' on affected side during motion.
